This invention relates to a process of purifying a fluorinated carbonyl compound which is expressed by the general formula (I), and more particularly to a process of separating the fluorinated compound (I) from a crude product gas containing hydrogen fluoride together with the compound (I) while recovering hydrogen fluoride. ##STR2## where x is 2 or 3 and A represents CF.sub.3 or H.
Typical examples of fluorinated carbonyl compounds of the general formula (I) are hexafluoroacetone (abbreviated to HFA), pentafluorochloroacetone (referred to as 5FK) and trifluoroacetoaldehyde (abbreviated to TFA), and these compounds are all industrially important intermediate materials for fluorine-containing polymers, medicines, agricultural chemicals, solvents, heat transfer media, etc.
Various methods are known for the synthesis of such compounds (I), but a prevailing and probably most favorable method is a vapor phase fluorination reaction between hydrogen fluoride HF and a chlorinated carbonyl compound, such as a perhaloacetone, which is taken as a precursor. The reaction is carried out at 300.degree.-400.degree. C. using a suitable catalyst such as chromium oxide. By this method a desired compound (I) can be obtained easily and with high yield, on condition that HF is used in great excess of a theoretical quantity, usually in a quantity two to three times the theoretical quantity. Furthermore, it has been proposed to increase the quantity of HF in the aforementioned reaction to about 4.5 times the theoretical quantity with a view to realizing very stable and long-lasting operating conditions. The use of a large excess of HF in the fluorination reaction results in the presence of a large amount of HF in the gaseous product of the reaction. Accordingly it becomes a matter of importance to separate HF from the fluorinated carbonyl compound.
The fluorinated carbonyl compounds (I) are known to form stable complexes with HF, as shown below by way of example:
(a) heptafluoroisopropanol (decomposition point 14.degree.-16.degree. C.) from HFA and HF, ##STR3##
(b) hexafluorochloroisopropanol (decomposition point 32.degree.-33.degree. C.) from 5FK and HF, ##STR4##
(c) 2-(1-chlorohexafluoroisopropoxy)-1-chloropentafluoroisopropanol (decomposition point -1.degree. C.) from 5FK and HF, ##STR5##
(d) tetrafluoro ethanol (b.p. 38.degree. C. at 780 mmHg) from TFA and HF, ##STR6## These complexes are stable compounds having relatively high decomposition points. At temperatures above the decomposition point of each complex, the gaseous product of the aforementioned fluorination reaction becomes an equilibrium mixture of the fluorinated compound (I), its HF complex and free HF. Therefore, a process of purifying the fluorinated compound (I) needs to include sole steps for the decomposition and/or separation of the complex.
In general, it is known to remove HF from the gaseous product of a fluorination reaction using excess HF by contact of the crude product gas with an inorganic fluoride such as NaF or CaF.sub.2 which adsorbs HF, and also it is known to remove HF by using a reaction between HF and an alkaline earth metal salt such as CaCO.sub.3 or CaCl.sub.2. However, neither of these methods is suited for industrial purification of the fluorinated compounds (I) because of needing large-scale purification apparatus. Besides, in the case of the latter method it is impossible to recover HF for reuse in the fluorination reaction.
Some different kinds of methods have been proposed for the decomposition and separation of the above described complexes. In one method, the gas containing the HF complex is absorped in water to decompose the complex and hydrate the organic components, and then HF existing in free state is neutralized with an alkali salt such as NaOH or Na.sub.2 CO.sub.3, followed by filtration to separate precipitated NaF. For the purification of the fluorinated compounds (I) this method is uneconomical because HF cannot be recovered and also because a portion of the compound (I) tends to decompose during the purifying procedure. In another method the complex is forced to react with sulfur trioxide in order to separate HF as fluorosulfonic acid, and in a still different method the complex is forced to react with acetic anhydride in order to separate HF as acetyl fluoride. However, these two methods are unsuited to the recovery of HF gas and disadvantageous in using sulfur trioxide or acetic anhydride which is expensive and inconvenient for handling.
It is well known that concentrated sulfuric acid absorbs HF readily and largely but scarcely dissolves HCl therein. Using such properties of sulfuric acid, it has been proposed (e.g. Japanese Patent Application Publcation No. 42(1967)-10202) to remove HF coexisting with a gaseous reaction product by absorption of HF in sulfuric acid.